Methods of manufacturing thin films of refractory dielectric materials



Apnl 12, 1960 K. J. FRANK 2,932,588

METHODS 0F MANUFACTURING THIN FILus A0F REFRACTORY DIELECTRIC uATx-:RIALS Filed June 22. 1956 T9-121 sa www@ xi; awsagaa l l:

United States Patent O ice METHODS F MANUFACI'URIN OF REFRACIORY DIELECIRIC MATERIAIS Kurt Josef Frank, Darmstadt, Germany, assignor to Englhh Electric Valve Company Limited, London, Eng- Ind, a British company Application Jaaa zz, 195s, sanar No. 593,198

emma priority, appuaaaop Graaf man my s, 195s z claim. (ci. 111-106) This invention relates to methods of manufacturing thin films and more particularly though not exclusively to thin films of refractory dielectric materials.

Oneknown method of creating a very thin film of a material is to place said material in, or on, a suitable container or holder and to raise the temperature of the material, in a vacuum, up to or beyond its evaporation temperature. The resultant evaporate is allowed to condense upon a suitable substrate so as to form a thin lilm deposit. This method is highly suited to certain materials, particularly some metals. Where, however, the material, has a very hi'gh evaporating temperature the substance from which the container or holder is made may also evaporate causing, inter alia, contamination of the condensate. Also, some materials, even when at temperature lower than their evaporation temperature, tend `chemically to react with the substance from which the container or holder is made, and this elect is also undesirable.

'I'he present invention seeks to provide means whereby materials, which upon being raised ,Ito their evaporation temperature as above described, would give rise to the aforementioned undesirable effects, may be satisfactorily evaporated substantially without said undesirable effects.

Although not limited exclusively thereto, the present invention is of great advantage when applied to the evaporation of refractory insulators of which 'silicon dioxide, alumina, magnesium oxide, and zirconium oxide are well known examples.

According to this invention in its broadest aspect a thin film of material is made by bombarding a specimen of the material in vacuo with an intense electron beam to evaporate material from said specimen and condensing the evaporate as a thin lilm on a carrier or substrate positioned in the vacuum.

Itis, of course, known to make holes in bodies by elec ironically bombarding said bodies to evaporate the material thereof where the holes are required.

In a preferred way of carrying outthe invention a thin lm of material is manufactured by producing a magnetic tlux path in an evacuated space, said path curving approximately through a right angle over its length, proecting a beam of electrons along said linx path so that the electron trajectory is bent to follow said path and the electrons are focussed on a specimen of material at the end of said path to cause evaporation of said material by electronic bombardment, and condensing the evaporate to form a thin film on `a carrier or substrate surface which is positioned clear of the magnetic linx and electron beam path. This preferred method has the great advantage that the electron gun producing the evaporating beam is practically entirely imune from having evaporate condensed thereon.

Preferably, where the material is an insulator, means are provided for regulating the velocity of the bombarding electrons to be substantially equivalent to that corresponding to the second cross-over potential on the secondsubstantially clear of the magnetic lines of liux between said poles for condensing the evaporate.

The invention is described with reference to the aocompanying drawings of which Fig. 1 shows a vertical cross-section of one form of apparatus for carrying out the invention and Fig. 2 is a horizontal cross section of a detail of Fig. l. Similar references are used for similar parts throughout the figures.

Referring to Fig. 1, a permanent magnet 1 arranged substantially in the shape of a horseshoe and designed so that the magnetic liux across its inter-pole space is of the order of 1,600 gauss or more, has one of its polepieces 2 adapted to receive or support a portion of rel fractory insulator material 3 to be evaporated. The other pole-piece 4 of said magnet comprises a hollow section 5, closed at one end, so that the inter-pole air gap extends from the external face of said closed end to the face of pole-piece 2. Dotted lines la indicate the general pattern of llux lines in the inter-pole air gap. An electron gun 6, of any well known kind, is centrally situated within section 5 and comprises a cathode 7 surrounded by a control cylinder or grid 8. A small tapered orifice 9, in the closed end of cylindrical section 5, allows electrons emitted by the electron gun to emerge, from the pole-piece.

'Ihe whole device, as described, is contained within an evaeuable transparent container shown as a belliar 10 on a platform 11. Suitable pumping arrangements, not shown, are connected to pipe 12, in platform 11, whereby to control the pressure within the bell-jar. Fig. 2 shows a horizontal cross-section of Fig. 1. taken along a line passing through the center of electron gun 6 substantially on line 6-6 of Fig. l. For clarity, bell-jar 10, platen 15, screen 16 and platform 11 are not included in this figure.

Magnet l, and therefore hollow section 5, is maintained through lead 5a at earth potential and where the material 3 is an insulator (the case presumed here) a negative potential of approximately 3 kilovolts applied via terminals 7b and 8a to cathode 7 and control grid 8 respectively. A small voltage, appropriate to the type of cathode used, is applied across terminals 7a and 7b to heat cathode 7. The actual value of said applied negative potential is preferably such that the velocity of electrons bombarding the material 3 will have an electron velocity substantially corresponding to the so-called second crossover potential on the secondary emission characteristic for the material to be evaporated. Selection of this velocity has the advantage of enabling a smaller anode potential to be used in the gun than would otherwise be the case. If the material 3 were a metal there would be, in general, no point in restricting the bombarding velocity the upper limit of bombardment velocity being set by practical considerations, principally the necessity of providing adequate insulation in the gun and the diliiculty of providing suliicient magnetic ux to maintain a high velocity beam properly focussed on the material. Line 6a shows the trajectory of electrons leaving the gun. After passing through orifice 9 the electrons will follow substantially the path of magnetic llux lines 1a and impinge upon the material 3 to be evaporated. The point of impingernent is made as small as possible in order that Patented Apr. 13, 1960 l uw .Y

the high current densities necessary to cause evaporation (of the order of 25 amps. per square centimeter) may be obtained.

On bombardment by electrons the material 3 will ex-A perience a rapid increase of temperature at the point of electron impingement and evaporation will occur from said point, the evaporate rising through the vacuum in the direction of arrow 13 to condense as a thin ilm on a maior surface 14 (the substrate) of a platen 15, suspended above the material 3. A shield 16, of non-magnetic material, is provided as a further safeguard to prevent any of the evaporate from condensing upon the face of polepiece 4. Such condensation would form on the wall of orifice 9 a coating which would allow a high electrostatic charge to be set-up on said face, and interfere with the electron beam.

After a suicient layer of refractory insulator material has been deposited upon substrate 14, the voltages applied to gun 6 are switched oti and platen 15 removed. The insulator material on the substrate will be in the form of a thin lm and experiment has shown that it is practically uncontaminated and required no further chemical treatment.

' 4 ganlcdlmegcollodiommountedonaringohmsterial whose expansion coefficient is matched to that of the glass. After the evaporation procesa the lilm is heated in air to a temperature at which the viscosity of the glass is such that the film will tighten by surface tension and the organic film will burn away, leaving the free supporting glass lm. A detailed description of the process is not given here as a full description is to be found in applicant's British patent specification No. 709,503.

Iclarm' 1. Apparatus for making thin tlms of material comprising within an evacuable chamber, a magnet of which one pole piece has its end face adapted to support the material to be evaporated and other pole piece surrounds an electron gun situated so as to re electrons through an orice in the face of said other pole piece, said face extending substantially at right angles to the end face of said rst mentioned one poley piece, and means situated Although the hereinbefore described technique is suitl able for short evaporation periods, over longer periods the electron beam will gradually drill a hole through the insulator material. It is therefore preferred, in auch cases, to arrange for the material 3 to be slowly moved in a plane at right angles to the electron path, so as to continuously bring fresh areas of the material under electronic bombardment.

For evporating glass, or other materials containing several components having dilerent evaporation temperatunes, in such a manner that the film condensed on the substrate will contain all these components intimately mixed with each other in the same proportion as in the original material, the following procedure may be employed. A sample of the material is supported on a carriage which can be moved in a plane at right angles to the electron path at the point of impingement, the movement being such that the sample is scanned under the electron.beam, at constant speed. This speed and the power density of the beam is so adjusted that the part of the material penetrated by the beam is raised, within the time of bombardment, to a temperature suliicient to evaporate all components, while the temperature of the remainder is not allowed to reach the evaporation point of the most volatile component. In this fashion layer after layer of the material is removed by evaporation.

Free supporting glass lms can be made by the method of the present invention using as a substrate an orsubstantially clear of the magnetic lines of ux between said poles for condensing the evaporate.

2. A method of making a thin tilm of material on a substrate consisting in producing a magnetic tlux path curving through approximately a right angle along its length in an evacuated space, projecting a beam of elec trons along said magnetic ux path so that the electron trajectory is bent to follow said magnetic ux path, supporting a specimen of material at the end of said mag. netic flux path, focussing said beam of electrons upon said specimen while simultaneously exposed to said magnetic ux path for effecting evaporation of said material by electronic bombardment and condensing the evaporate in a position external to both the magnetic tlux path and the electron trajectory to form a thin lm on said substrate.

References Cited in the tile of this patent UNITED STATES PATENTS 2,103,623 Kott Dec. 28, 1937 2,157,478 Burkhardt et al. May 9, 1939 2,239,642 Burkhardt et al Apr. 22, 1941 2,527,747 Lewis et al Oct. 31, 1950 2,754,259 Robinson et al. July l0, 1956 2,771,568 Steigerwald Nov. 20, 1956 FOREIGN PATENTS 754,102 Great Britain Aug. l, 1956 OTHER REFERENCES 

2. A METHOD OF MAKING A THIN FILM OF MATERIAL ON A SUBSTRATE CONSISTING IN PRODUCING A MAGNETIC FLUX PATH CURVING THROUGH APPROXIMATELY A RIGHT ANGLE ALONG ITS LENGTH IN AN EVACUATED SPACE, PROJECTING A BEAM OF ELECTRONS ALONG SAID MAGNETIC FLUX PATH SO THAT THE ELECTRON TRAJECTORY IS BENT TO FOLLOW SAID MAGNETIC FLUX PATH, SUPPORTING A SPECIMEN OF MATERIAL AT THE END OF SAID MAGNETIC FLUX PATH, FOCUSSING SAID BEAM OF ELECTRONS UPON SAID SPECIMEN WHILE SIMULTANEOUSLY EXPOSED TO SAID MAGNETIC FLUX PATH FOR EFFECTING EVAPORATION OF SAID MATERIAL BY ELECTRONIC BOMBARDMENT AND CONDENSING THE EVAPORATE IN A POSITION EXTERNAL TO BOTH THE MAGNETIC FLUX PATH AND 